1. Field of the Invention
The present invention relates to an implantable multipole coaxial lead or cable of the type suitable for use in an electrode arrangement for stimulating intracardiac tissue in a pacer system.
2. Description of the Prior Art
The demands on a multi-pole coaxial lead for use as part of an implanted electrode arrangement in systems such as heart pacemakers with a long life time are extremely high. A lead of this type must be body compatible, of low resistance to maintain energy consumption at a low level thereby increasing life time of pacemaker batteries, exhibit a high fatigue strength under repeated bending stresses, pliant in order to cause minimum disruptions to surrounding organs, as well as stiff enough when pushed forward to enable an easy insertion by the physician.
Moreover, these demands are at least in part contradicting as is obviously the case for pliant and low resistance requirements. In order to be pliant the conductors of the lead should essentially have a small diameter and in order to be of low resistance the conductors should have a larger diameter.
The attempts to overcome these difficulties in the past are manifold. A coaxial lead is disclosed in German OS No. 3 031 752 wherein the conductors are individually insulated and wound into a single multi-pole helix. Although the diameter of this lead is reduced in comparison to a lead with an inner and an outer helix of conductors separated from another by an insulating sheath and the conductors also being surrounded at the exterior of the lead by a further insulating sheath, the multi-pole helix necessarily has a steeper pitch and thus exhibits reduced flexibility. Further, the single insulating material requires a compromise between the electric properties and body compatibility for that material.
In European Application No. 92 798 a lead is disclosed wherein the outer helix is a metal tape or tape-shaped cord in order to reduce diameter and resistance while increasing flexibility.
A multi-pole lead for catheters and electrical measurements in the medical field is exemplified in German OS No. 2 408 707 in which a multi-pole helix of axially separated conductors with a first insulating material is further insulated by disposing the helix in the wall of a tube-formed second insulating material. The interior of the tube is a fluid conduit. Although this arrangement where the conductors and the first insulating material are embedded in the second insulating material achieves having a reduced diameter as well as good electrical insulating and body compatibility properties, the explicitly stated materials for the conductor, copper, and for the first insulating material, insulating paint, have properties which make this lead unsuitable for implantation, especially intracardial implantation. The materials in a lead for intracardial implantation must withstand around 100,000 flexes a day, and consequently the lead must exhibit an extremely high fatigue strength, which copper does not satisfy. Further, if a stylet were used at insertion, insulating paint is unsuitable because the walls of insulating material in the central opening of the lead could easily be penetrated.